This invention relates to a method of manufacturing semiconductor laser devices, and more particularly to a method of bonding a semiconductor laser element to a heat sink.
A semiconductor laser element composed of one of the following families of compound semiconductor materials, the gallium-arsenic (GaAs) family, the gallium-aluminum-arsenic (GaAlAs) family or the indium-gallium-arsenic-phosphor (InGaAsP) family, is currently being considered for practical applications for optical communication systems.
Such semiconductor laser elements can be practically applied to optical communication systems. These systems are required to have a high output power, long operational life and high reliability. In order to meet such requirements, it is essential to improve electrical characteristics of said semiconductor laser element and simultaneously efficiently release the Joule heat which is generated from the active layer of said semiconductor laser element being driven by a high current density.
As a means for improving dispersion of heat, it is proposed to use a diamond (Type IIa diamond), having a thermal conductivity about 5 times higher than that of copper at room temperature, as a heat sink material. The diamond replaces the metallic materials such as copper (Cu) or silver (Ag), etc., currently used as heat sink materials.
When mounting and bonding a semiconductor laser element on a diamond heat sink, it is required to mount and bond such semiconductor laser element without structurally degrading it, while ensuring satisfactory thermal and electrical coupling between the heat sink and the semiconductor laser element.
A diamond heat sink facilitates bonding and support of a semiconductor laser element with a metallized layer formed on the element's surface and provides the one lead-out electrode of said semiconductor laser element.
Certain conventional technological methods have been proposed to mount and bond a semiconductor laser element to a diamond heat sink on which surface a metallized layer is formed.
In other words, a first proposed method provides that a gold (Au) layer be formed as the metallized layer on the surface of said diamond heat sink and a semiconductor laser element is bonded to said gold layer by indium (In) soldering. This method is proposed because the mechanical distortion generated when the semiconductor laser element is bonded to a heat sink greatly affects the operating life of said semiconductor laser element. Bonding by means of the indium solder minimizes the mechanical distortion.
As a second method, it is proposed that a tin (Sn) layer be formed as the metallized layer on the surface of said diamond heat sink and a gold layer be formed on the bonding surface of the semiconductor laser element to be bonded therein and both the tin layer and the gold layer placed in contact and heated. Thus a gold-tin family eutectic crystalline alloy is generated, thereby bonding a semiconductor laser element to a diamond heat sink.
As a third method, it is proposed that a layer consisting of gold-antimony (Sb) or one of tin or indium-gold is formed on the surface of a diamond heat sink as the metallized layer, and a chromium (Cr)-gold layer is formed on the bonding surface of a semiconductor laser element. Both metallized layers are then placed in contact and heated under a pressured condition, thus the gold-antimony family eutectic crystalline alloy or gold-tin family eutectic crystalline alloy or gold-indium family eutectic crystalline alloy is generated, and the semiconductor laser element is bonded to the diamond heat sink.
As examples, the second and third methods are employed for analysis of thermal resistance, electrical resistance or mechanical properties of the semiconductor laser device when bonding by means of the gold family eutectic crystalline alloy is employed.
However, in the bonding structure or bonding means using the conventional indium solder of the first method, the bonding condition varies with age, particularly, the thermal resistance increases distinctively and therefore the drive current must be increased drastically to maintain the laser beam output level. This significantly reduces the operating life of the relevant laser element.
In the second method, where a gold layer is formed on the bonding surface of a semiconductor laser element, and a tin layer is formed on the surface of the heat sink and both layers are placed in contact and heated, the tin layer surface is likely to be oxidized before being placed in contact and heated. Therefore, satisfactory thermal and mechanical coupling between the semiconductor laser element and the heat sink cannot be obtained even if they are satisfactorily placed in contact and heated.
The third method presents a means to overcome this problem of the second method. Since the metallized layer formed on the surface of the heat sink has the multi-layered structure of, for example, gold-tin-gold or tin-gold where the tin (or antimony or indium) is not exposed and is covered with the gold, the tin (or antimony or indium) is not oxidized before the semiconductor laser element and heat sink are placed in contact and heated. However, this method requires an additional step, that is, the gold layer is formed covering the tin (or antimony or indium). This increases manufacturing processes. Therefore, such a method is not suitable for mass-production and has reduced desirability for producing less costly products.